Motor with adjustable back-electromotive force

ABSTRACT

The described apparatus and method enable alteration of motor properties during operation of a motor. For example, the rotor of the motor may be adjustable, during motor operation, between a first diameter and a larger, second diameter. When the diameter of the rotor increases, the distance between the electromagnetic coils of the stator and the magnets of the rotor increases, thereby reducing the back-electromotor force (back-EMF) of the motor. When the back-EMF of the motor decreases, the torque of the motor decreases but the maximum revolutions per minute (RPM) increases. When the diameter of the rotor decreases, the distance between the electromagnetic coils of the stator and the magnets of the rotor decreases, thereby increasing the back-EMF of the motor. When the back-EMF of the motor increases, the torque of the motor increases but the maximum RPM decreases.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority to U.S.application Ser. No. 15/268,390, filed Sep. 16, 2016, which isincorporated herein by reference in its entirety.

BACKGROUND

Unmanned aerial vehicles are continuing to increase in use. For example,unmanned aerial vehicles are often used for surveillance. While thereare many beneficial uses of unmanned aerial vehicles, they also havemany drawbacks. For example, many unmanned aerial vehicles utilizemultiple motors and propellers to maintain flight and navigate. Forexample, some unmanned aerial vehicles may include four motors andpropellers, referred to as quad-copters, eight motors and propellers,referred to as octo-copters, etc. Utilizing multiple motors increasesthe sound generated by the unmanned aerial vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number appears.

FIG. 1 depicts a block diagram of a top-down view of an unmanned aerialvehicle, according to an implementation.

FIG. 2 depicts a detailed view of a motor stator and the magnets of therotor of the motor, according to an implementation.

FIG. 3A depicts a top-down view of a motor stator and a rotor of aoutrunner brushless motor, according to an implementation.

FIG. 3B depicts a view of a sensor and controller plate of the outrunnerbrushless motor illustrated in FIG. 3A, according to an implementation.

FIG. 4 is a flow diagram illustrating an example motor drive process,according to an implementation.

FIG. 5A depicts a top-down view of a motor stator and a rotor in acontracted position, according to an implementation.

FIG. 5B depicts a top-down view of the motor stator and rotor of FIG. 5Awith the rotor in an expanded position, according to an implementation.

FIG. 6A depicts a rotor in a contracted position, according to animplementation.

FIG. 6B depicts the rotor of FIG. 6A in an expanded position, accordingto implementation.

FIG. 6C depicts a rotor in a contracted position, according to animplementation.

FIG. 6D depicts the rotor of FIG. 6C in an expanded position, accordingto implementation.

FIG. 7 is a flow diagram illustrating an example motor propertyadjustment process, according to an implementation.

FIG. 8 is a block diagram illustrating various components of an unmannedaerial vehicle control system, according to an implementation.

While implementations are described herein by way of example, thoseskilled in the art will recognize that the implementations are notlimited to the examples or drawings described. It should be understoodthat the drawings and detailed description thereof are not intended tolimit implementations to the particular form disclosed but, on thecontrary, the intention is to cover all modifications, equivalents andalternatives falling within the spirit and scope as defined by theappended claims. The headings used herein are for organizationalpurposes only and are not meant to be used to limit the scope of thedescription or the claims. As used throughout this application, the word“may” is used in a permissive sense (i.e., meaning having the potentialto), rather than the mandatory sense (i.e., meaning must). Similarly,the words “include,” “including,” and “includes” mean “including, butnot limited to.”

DETAILED DESCRIPTION

This disclosure describes methods and apparatus for altering the soundgenerated by a motor during operation. For example, rather than equallyspacing magnets of a rotor of a brushless direct current (DC) motorequally about the rotor, the implementations described herein include amotor in which the spacing between the rotor magnets is non-uniform(i.e., the spacing varies between rotor magnets). With non-uniformspacing of the rotor magnets, the sound generated during operation ofthe motor is altered. In some implementations, the non-uniform spacingmay be a small amount such that traditional electronic speed controllers(ESC) may continue to be operable and control rotation of the motor. Inother implementations, the motor may include one or more sensors and/orcontrollers that monitor a position of the rotor, determine currentpatterns for different electromagnetic coils of the stator of the motor,and/or send different currents to different electromagnetic coils of themotor to control operation of the motor. In still other implementations,the spacing or alignment of the electromagnetic coils of the stator mayalso be non-uniform or irregularly spaced.

In addition to altering the sound of the motor through non-uniformspacing of the rotor magnets, non-uniform distribution of theelectromagnetic coils, and/or controlling the current patterns todifferent electromagnetic coils, in some implementations, one or moreproperties of the motor may be altered during operation of the motor.For example, the rotor of the motor may be adjustable, during motoroperation, between a first diameter and a larger, second diameter. Whenthe diameter of the rotor increases, the distance between theelectromagnetic coils of the stator and the magnets of the rotorincreases, thereby reducing the back-electromotor force (back-EMF) ofthe motor. When the back-EMF of the motor decreases, the torque of themotor decreases but the maximum revolutions per minute (RPM) increases.When the diameter of the rotor decreases, the distance between theelectromagnetic coils of the stator and the magnets of the rotordecreases, thereby increasing the back-EMF of the motor. When theback-EMF of the motor increases, the torque of the motor increases butthe maximum RPM decreases.

During aerial navigation of an aerial vehicle, such as an unmannedaerial vehicle (UAV), the desired operational properties of the motorsmay vary. For example, during takeoff, landing, or while maneuvering inparticular areas, higher motor torque may be desirable over a highermaximum RPM. In comparison, during transit flight at a high altitude,higher maximum RPM may be more desirable than higher torque. By alteringthe diameter of the rotor of the motor, thereby increasing or decreasingthe back-EMF of the motor, the torque and maximum RPM is likewiseadjusted.

FIG. 1 illustrates a block diagram of a top-down view of an UAV 100,according to an implementation. As illustrated in FIG. 1, the UAV 100includes eight propellers 102-1, 102-2, 102-3, 102-4, 102-5, 102-6,102-7, 102-8 powered by motors and spaced about a body 104 of the UAV aspart of a propulsion system. Details of the motors are provided below.

The motors and propellers 102 may be of any type and of a sizesufficient to lift the UAV 100 and any items engaged by the UAV 100 sothat the UAV 100 can navigate through the air, for example, to deliveran item to a location. The propellers may be made of one or moresuitable materials such as graphite, carbon fiber, etc. The propellersmay be fixed or variable pitch propellers. Likewise, in someimplementations, the motors and/or motor arms may be configured suchthat an angle or orientation of the motor with respect to the UAV may bealtered. While the example of FIG. 1 includes eight motors andpropellers, in other implementations, more or fewer motors and/orpropellers may be utilized for the propulsion system of the UAV 100.Likewise, in some implementations, the motors and/or propellers may bepositioned at different locations on the UAV 100. In addition,alternative methods of propulsion may be utilized. For example, engines,fans, jets, turbojets, turbo fans, jet engines, and the like may be usedto propel the UAV.

The body 104 or frame of the UAV 100 may be of any suitable material,such as graphite, carbon fiber, and/or aluminum. In this example, thebody 104 of the UAV 100 includes four rigid members 105-1, 105-2, 105-3,105-4, or beams arranged in a hash pattern with the rigid membersintersecting and joined at approximately perpendicular angles atintersection points 107-1, 107-2, 107-3 and 107-4. The propellers 102and corresponding motors are positioned at both ends of each rigidmember 105. In this example, rigid members 105-1 and 105-3 are arrangedparallel to one another and are approximately the same length. Rigidmembers 105-2 and 105-4 are arranged parallel to one another, yetperpendicular to rigid members 105-1 and 105-3. Rigid members 105-2 and105-4 are approximately the same length. In some embodiments, all of therigid members 105 may be of approximately the same length while, inother implementations, some or all of the rigid members may be ofdifferent lengths. Likewise, the spacing between the two sets of rigidmembers may be approximately the same or different.

While the implementation illustrated in FIG. 1 includes four rigidmembers 105 that are joined to form at least part of the body 104, inother implementations, there may be fewer or more components to the body104. For example, rather than four rigid members, in otherimplementations, the body 104 of the UAV 100 may be configured toinclude six rigid members. In such an example, two of the rigid members105-2, 105-4 may be positioned parallel to one another. Rigid members105-1, 105-3 and two additional rigid members on either side of rigidmembers 105-1, 105-3 may all be positioned parallel to one another andperpendicular to rigid members 105-2, 105-4. With additional rigidmembers, additional cavities with rigid members on all four sides may beformed by the body 104. As discussed further below, a cavity within thebody 104 may be configured to include an engagement mechanism 134 forthe engagement and transport of item(s) and/or containers that containitem(s) (e.g., for the delivery of an ordered item to a user).

In some implementations, the UAV may be configured for aerodynamics. Forexample, an aerodynamic housing may be included on the UAV that enclosesthe UAV control system 130, one or more of the rigid members 105, thebody 104, and/or other components of the UAV 100. The housing may bemade of any suitable material(s) such as graphite, carbon fiber,aluminum, etc. Likewise, in some implementations, the engagementmechanism 134 may be configured such that, when an item is engaged, itis enclosed within the frame and/or housing of the UAV 100 so that noadditional drag is created during transport of the item by the UAV 100.

Extending outward from each rigid member is a support arm 106 that isconnected to a safety barrier 108. In this example, the safety barrieris positioned around and attached to the UAV 100 in such a manner thatthe motors and propellers 102 are within the perimeter of the safetybarrier 108. The safety barrier may be plastic, rubber, etc. Likewise,depending on the length of the support arms 106 and/or the length,number or positioning of the rigid members 105, the safety barrier maybe round, oval, or any other shape.

Mounted to the body 104 is the UAV control system 130. In this example,the UAV control system 130 is mounted in the middle and on top of thebody 104. The UAV control system 130, as discussed in further detailbelow with respect to FIG. 8, controls the navigation, communication andother operations of the UAV 100. In various implementations, the UAV 100may also include one or more power modules 132. In this example, the UAV100 includes two power modules 132 that are removably mounted to thebody 104. In various implementations, the power module(s) for the UAVmay be in the form of battery power, solar power, gas power, supercapacitor, fuel cell, alternative power generation source, or acombination thereof. The power modules 132 are coupled to and providepower for the UAV control system 130, the motors, and/or othercomponents of the UAV 100.

As noted above, the UAV may also include an engagement mechanism 134.The engagement mechanism 134 may be configured to engage and disengageitems and/or containers that hold items. In this example, the engagementmechanism 134 is positioned within a cavity of the body 104 that isformed by the intersections of the rigid members 105. The engagementmechanism 134 may be positioned beneath the UAV control system 130. Inimplementations with additional rigid members, the UAV may includeadditional engagement mechanisms and/or the engagement mechanism 134 maybe positioned in a different cavity within the body 104. The engagementmechanism 134 may be of any size sufficient to securely engage anddisengage items and/or containers that contain items. The engagementmechanism 134 communicates with (e.g., via wired or wirelesscommunication) and is controlled by the UAV control system 130.

While the implementations of the UAV discussed herein utilize propellersto achieve and maintain flight, in other implementations, the UAV may beconfigured in other manners. For example, the UAV may also include fixedwings and/or a combination of both propellers and fixed wings. In suchconfigurations, the UAV may utilize one or more propellers to enabletakeoff and landing and a fixed wing configuration or a combination wingand propeller configuration to sustain flight while the UAV is airborne.

FIG. 2 depicts a detailed view of a outrunner brushless direct current(DC) motor 200 and the magnets 212 of the rotor 204 of the motor 200,according to an implementation. An outrunner brushless DC motortypically includes a base 208, a stator 209, and a rotor 204. The base208 is generally used to affix the motor 200 to an aerial vehicle, suchas a UAV. Likewise, the stator 209 is coupled to the base. The stator209, also known as an armature, includes an electromagnetic assembly210, and is typically configured in a cylindrical manner, as shown inFIG. 2, and remains stationary on the base.

The rotor 204 is also configured in a cylindrical manner such that itextends above the base 208 and a housing 205 of the rotor 204 forms acavity that substantially encompasses and rotates around theelectromagnetic assembly 210 of the stator 209. On an interior surfaceof the rotor housing 205 are a series of magnets 212 that are used todrive rotation of the rotor. Specifically, as is known in the art, whena current is applied to the electromagnetic coils 210, it causesalternating polarities of the electromagnetic coils which attract orrepel the magnets 212 affixed to the interior surface of the rotorhousing 205. The attraction/repulsion of the magnets 212 by theelectromagnetic coils 210 of the stator 209 cause the rotor 204 torotate. A propeller 202 is also affixed to the rotor 204 and rotateswith the rotor 204.

As illustrated in the expanded view of FIG. 2, the magnets 212 areaffixed to the interior surface of the rotor housing 205. In thedescribed implementations, the spacing between the magnets isnon-uniform. For example, the spacing between the plurality of magnets212 may be random, in a repeating pattern, or other configuration suchthat the spacing between two magnets is different than the spacingbetween at least two other magnets 212 of the rotor 204. In someimplementations, the spacing between the magnets 212 is such that no twospaces are the same. In other implementations, the spacing is such thatspacing between each adjacent pair of magnets is different. For example,the distance d_(R1) is different than the distance d_(R2). Likewise, thedistance d_(R3) may be different than the distances d_(R1) and d_(R4)but may be the same or different than the distance d_(R2). In otherimplementations, each of the distances d_(R1), d_(R2), d_(R3), andd_(R4) may be different. The “distance” between two magnets of therotor, as used herein, may be a linear distance or a circumferentialdistance. For example, the distance d_(R1) between two adjacent rotormagnets 212 may be a first circumferential distance and the distanced_(R2), which is different than d_(R1), between two other adjacent rotormagnets 212, may be a second circumferential distance.

In some implementations, in addition to and/or as an alternative tovarying the spacing between the magnets 212 of the rotor, the spacingbetween the plurality electromagnetic coils 210 of the stator 209 may bevaried such that the spacing of the electromagnetic coils isnon-uniform. For example, the spacing between the plurality ofelectromagnetic coils 210 may be random, in a repeating pattern, or anyother configuration such that the spacing between two electromagneticcoils is different than the spacing between at least two otherelectromagnetic coils 210 of the stator 209. In some implementations,the spacing between the electromagnetic coils 210 is such that no twoelectromagnetic coils are the same distance apart. In otherimplementations, the spacing is such that the spacing between eachadjacent pair of electromagnetic coils is different. For example, thedistance d_(S1) is different than the distance d_(S2). Likewise, thedistance d_(S3) may be different than the distance d_(S2) but may be thesame or different than the distance d_(S1). In other implementations,each of the distances d_(S1), d_(S2), and d_(S3) may be different.

By varying the spacing of the magnets 212 of the rotor and/or thespacing of the electromagnetic coils 210 of the stator 209, the soundgenerated by the motor during operation is altered. In traditionaloutrunner brushless DC motors, during operation, the motor generates aperiodic noise. By irregularly spacing the magnets of the rotor and/orthe electromagnetic coils of the stator, the resultant sound duringoperation is non-periodic, or random (e.g., there is no discreet ordominant tone). In some implementations, the resulting sound of motorsdesigned according to the implementations discussed herein isrepresentative of a broadband noise, such as white noise.

FIG. 3A depicts a top-down view of a motor stator 309 and a rotor 304 ofan outrunner brushless motor, according to an implementation. In thisexample, there are nine magnets 312 that are irregularly spaced aboutthe interior surface of the rotor 304. Likewise, the stator 309 includessix electromagnetic coils 302-1, 304-1, 306-1, 308-1, 310-1, 313-1 thatare irregularly spaced. As discussed above, the spacing of the magnets312 about the interior of the rotor may be such that distance betweeneach magnet is different, that no two adjacent distances are the same,the varying distances may have repeating patterns, etc.

In one implementation, a first magnet 312-1 and a second magnet 312-2are separated by a first distance d_(R1) and the second magnet 312-2 anda third magnet 312-3 are separated by a second distance d_(R2) that isdifferent than the first distance d_(R1). Likewise, the third magnet312-3 and a fourth magnet 312-4 are separated by a third distanced_(R3), the fourth magnet 312-4 and a fifth magnet 312-5 are separatedby a fourth distance d_(R4), the fifth magnet 312-5 and a sixth magnet312-6 are separated by a fifth distance d_(R5), the sixth magnet 312-6and a seventh magnet 312-7 are separated by a sixth distance d_(R6), theseventh magnet 312-7 and an eighth magnet 312-8 are separated by aseventh distance d_(R7), the eighth magnet 312-8 and a ninth magnet312-9 are separated by an eighth distance d_(R8), and the ninth magnet312-9 and the first magnet 312-1 are separated by a ninth distanced_(R9).

In accordance with the described implementations, at least two of thedistances d_(R1), d_(R2), d_(R3), d_(R4), d_(R5), d_(R6), d_(R7),d_(R8), and d_(R9) are different. In some implementations, all of thedistances may be different. In other implementations, the varyingdistances may be arranged in a repeating pattern. For example, the firstmagnet 312-1 and a second magnet 312-2 may be separated by the firstdistance d_(R1), the second magnet 312-2 and the third magnet 312-3 maybe separated by the second distance d_(R2) that is different than thefirst distance d_(R1), the third magnet 312-3 and the fourth magnet maybe separated by a third distance d_(R3) that is different than the firstdistance d_(R1) and the second distance d_(R2), the fourth magnet 312-4and the fifth magnet 312-5 may be separated by the first distanced_(R1), the fifth magnet 312-5 and the sixth magnet 312-6 may beseparated by the second distance d_(R2), the sixth magnet 312-6 and theseventh magnet 312-7 may be separated by the third distance d_(R3), theseventh magnet 312-7 and the eighth magnet 312-8 may be separated by thefirst distance d_(R1), the eighth magnet 312-8 and the ninth magnet312-9 may be separated by the second distance d_(R2), and the ninthmagnet 312-9 and the first magnet 312-1 may be separated by the thirddistance d_(R1). By having such a repeating pattern, upon detection ofone of the distances, a relative position of the rotor with respect tothe electromagnetic coils can be determined.

In yet another example, the separation of the magnets may include arepeating pattern, such as the one illustrated above, with an additionaldistance between two magnets that is different than any of the distancesof the repeating pattern. The unique distance may be used as a positiondeterminant that when detected indicates an absolute position of therotor of the motor.

As discussed further below, the variability in the distance between themagnets may be just a few millimeters. As the rotor rotates, the one ormore sensors, such as a Hall sensor, may be positioned on the base ofthe motor to detect a presence or absence of a magnet as the magnetspass over or past the sensors. In the implementation illustrated inFIGS. 3A and 3B, a sensor may be included on the base of the motor andadjacent each electromagnetic coil 302 of the stator such that thesensor can detect a presence of an approaching magnet before the magnetreaches the corresponding electromagnetic coil. For example, a firstsensor 302-2 may be positioned adjacent a first electromagnetic coil302-1, a second sensor 304-2 may be positioned adjacent a secondelectromagnetic coil 304-1, a third sensor 306-2 may be positionedadjacent a third electromagnetic coil 306-1, a fourth sensor 308-2 maybe positioned adjacent a fourth electromagnetic coil 308-1, a fifthsensor 310-2 may be positioned adjacent a fifth electromagnetic coil310-1, and a sixth sensor 313-2 may be positioned adjacent a sixthelectromagnetic coil 313-1.

As the rotor rotates, each of the sensors detect a presence or absenceof a magnet 312 of the rotor as the magnets pass over the sensors. Ateach detection of a magnet, the sensor may send an indication of thedetected presence of the magnet. The indication may be sent to acontroller and the respective controller may determine a current patternto apply to an electromagnetic coil 309 to either retract or repel themagnet as it passes the electromagnetic coil. In some implementations,each of the sensors 302-2, 304-2, 306-2, 308-2, 310-2, and 313-2 may becoupled to a respective controller 302-3, 304-3, 306-3, 308-3, 310-3,and 313-3.

As illustrated in FIG. 3B, the controllers 302-3, 304-3, 306-3, 308-3,310-3, and 313-3 may likewise be included in the base of the motor andbe configured to control and/or send currents to respectiveelectromagnetic coils. For example, each controller 302-3, 304-3, 306-3,308-3, 310-3, and 313-3 may be configured to provide one or more currentand/or current patterns to a respective electromagnetic coil 302-1,304-1, 306-1, 308-1, 310-1, and 313-1.

For example, the first sensor 302-2 may detect a presence of a magnet312 as the magnet passes over or by the sensor 302-2. The sensor, inresponse to detecting the magnet sends an indication to the controller302-3. The controller, upon receiving an indication of the presence ofthe magnet causes a current to be applied to the electromagnetic coil302-1 that will cause the detected magnet to be attracted toward theelectromagnetic coil or repelled away from the electromagnetic coil. Insome implementations, the sensor may also determine based on indicationsreceived from the sensor a distance between two detected magnets. Forexample, the sensor may receive an indication from the UAV controlsystem indicating a commanded rotational speed for the motor. Based onthe commanded rotation speed and a time between indications from thesensor indicating the presence of magnets, the controller can determinea distance between the two detected magnets. Depending on theconfiguration of the motor, the determined distance may be used todetermine a relative position of the rotor (e.g., for a repeatingpattern) or an absolute position of the rotor (e.g., for a rotor inwhich each distance is distinct, or upon detection of a uniquedistance).

While the above example describes the use of a sensor and controller foreach of the electromagnetic coils, in other implementations, fewer oradditional sensors may be utilized. For example, the UAV control systemand/or a controller used to provide current to the electromagnetic coilsof the motor may contain motor configuration information indicating thespacing distances between each magnet of the rotor and/or the spacingand position of the electromagnetic coils. Upon determination by one ormore sensors of a spacing between two or more magnets, a relative orabsolute position of the rotor, based on the known magnet configuration,can be determined. Based on the determined position of the rotor and theknown position of the magnets and electromagnetic coils of the motor,current patterns may be selected from a defined table maintained in theUAV data store (discussed below) and sent to the respectiveelectromagnetic coils to control operation of the motor.

For example, if the separation distance between each magnet of a rotoris different, upon detection by a single sensor of two magnets and adetermination of the distance between those magnets, the absoluteposition of the rotor and each magnet of the rotor can be determined.Based on the determined positions, different current patterns may besent to each of the electromagnetic coils of the motor. In a similarexample, if there are three different separation distances that repeataround the rotor, upon detection of one of the three separationdistances, a relative position of the rotor can be determined andcurrent patterns sent to the electromagnetic coils based on thedetermined position of the rotor.

As still another example, if the different spacing distances include atleast one unique distance, upon detection of the unique distance betweentwo magnets the absolute position of the rotor can be determined from asingle sensor.

FIG. 4 is a flow diagram illustrating an example motor drive process400, according to an implementation. This process, and each processdescribed herein, may be implemented by the architectures describedherein or by other architectures. The process is illustrated as acollection of blocks in a logical flow graph. Some of the blocksrepresent operations that can be implemented in hardware, software, or acombination thereof. In the context of software, the blocks representcomputer-executable instructions stored on one or more computer readablemedia that, when executed by one or more processors, perform the recitedoperations. Generally, computer-executable instructions includeroutines, programs, objects, components, data structures, and the likethat perform particular functions or implement particular abstract datatypes.

The computer readable media may include non-transitory computer readablestorage media, which may include hard drives, floppy diskettes, opticaldisks, CD-ROMs, DVDs, read-only memories (ROMs), random access memories(RAMs), EPROMs, EEPROMs, flash memory, magnetic or optical cards,solid-state memory devices, or other types of storage media suitable forstoring electronic instructions. In addition, in some implementations,the computer readable media may include a transitory computer readablesignal (in compressed or uncompressed form). Examples of computerreadable signals, whether modulated using a carrier or not, include, butare not limited to, signals that a computer system hosting or running acomputer program can be configured to access, including signalsdownloaded through the Internet or other networks. Finally, the order inwhich the operations are described is not intended to be construed as alimitation, and any number of the described operations can be combinedin any order and/or in parallel to implement the process.

The example process 400 begins upon receipt of a motor rotation speedcommand, as in 402. For example, the navigation system of the UAV and/ora remote control system may send a navigation command commanding the UAVto aerially navigate in a direction. The received command is convertedinto a motor command that includes a motor rotation speed necessary forthe motor to execute the navigation command in conjunction with othermotors of the UAV.

The example process 400 then determines the position of the rotor, as in404. As discussed above, the position of the rotor may be determinedusing one or more sensors that detect the presence or absence of magnetsof the rotor. Based on at least two detected magnets and a currentrotational speed of the motor, a distance between the magnets may bedetermined. Based on the measured distance between the magnets, theposition of the rotor can be determined. For example, motorconfiguration information, such as the distance of spacing between therotor magnets may be maintained in a data store. Using the measureddistance and the stored motor configuration information, the position ofthe rotor may be determined.

Upon determining the position of the rotor and known motor configurationinformation, a current pattern for one or more of the electromagneticcoils of the stator of the motor may be determined, as in 406. Forexample, the data store may maintain current pattern information that isto be applied to achieve different RPM from the motor. A current patternmay be a series of currents that are to be applied to theelectromagnetic coil of the motor during one revolution of the rotor.Based on the position of the rotor and a commanded RPM, the selectedcurrent pattern may be initiated at a specific point to correspond withthe position of the rotor magnets with respect to the electromagneticcoils of the stator of the motor.

Based on the selected current pattern, the current pattern is applied toone or more of the electromagnetic coils of the motor, as in 408. Insome implementations, the same current pattern may be applied to each ofthe electromagnetic coils, phase shifted based on the position of therotor magnets. Alternatively, different current patterns may be appliedto one or more of the electromagnetic coils based on the position of therotor magnets. In some implementations, different current patterns mayresult in the motor producing different output sounds during operationof the motor. Likewise, in some implementations, a current pattern froma first electromagnetic coil of the stator may generate a first soundoutput and a current pattern from a second electromagnetic coil of thestator may generate a second sound output. Likewise, the first soundoutput may cause interference with the second sound output, therebyaltering a total sound generated during operation of the motor. In someimplementations, the interference may be destructive interference thatreduces the total sound output from the motor during operation of themotor.

The example process 400 continues during operation of the motor withpotentially a different current pattern for each electromagnetic coilbeing selected and produced for each revolution of the motor.Accordingly, as the current pattern(s) is applied to the electromagneticcoil(s) the example process 400 returns to block 402 and continuesduring motor operation. If the commanded RPM and/or other forces (e.g.,wind) are acting upon the aerial vehicle, the current pattern may varyfor each rotation to alter the rotation of the motor and thus the soundproduced by the motor.

While the above examples have been discussed with respect to anoutrunner brushless DC motor, the implementations are equally applicableto an inrunner brushless DC motor in which the stator surrounds therotor and the rotor rotates within a cavity formed by a stationarystator.

In addition to altering sound generated from operation of a motor, asdiscussed above, in some implementations, one or more motor propertiesmay be altered during operation of the motor. For example, the back-EMFof a motor may be altered during motor operation by altering thediameter of the rotor of the motor. Such motor property modification maybe utilized in motors having irregularly spaced magnets, as discussedabove, or in motors with evenly spaced magnets.

FIG. 5A depicts a top-down view of a motor stator 509 and a rotor 504 ina contracted position, according to an implementation. As illustrated,the diameter (Diameter₁) of the rotor 504 in a contracted positionresults in the rotor magnets 512 being close to the electromagneticcoils 510 of the stator 509 such that the motor has a high torque. Thetorque is directly correlated to the back-EMF of the motor. Back-EMFrefers to the voltage that occurs in electric motors where there is arelative motion between the rotor and the magnetic field from theelectromagnetic coils of the stator. The smaller the gap (D_(G1))between the stator 510 and the rotor magnets the higher the back-EMF andthus the higher the torque of the motor.

In addition to the rotor having a gap distance (D_(G1)) between therotor motors 512 and the electromagnetic coils 510, the rotor magnetsare also separated by a contracted distance, referred to in the expandedview of FIG. 5A as distance D_(R1). As noted above, the distance betweenthe rotor magnets may be uniform or irregular. Regardless, in thecontracted configuration illustrated in FIG. 5A, each of the rotormagnet are separated by a distance (D_(R1)), generally referred toherein as a contracted distance. For example, rotor magnet 512-1 androtor magnet 512-2 are separated by a distance D_(R1).

While a higher back-EMF produces more torque for the motor, it alsoresults in a lower maximum RPM for the motor and requires additionalpower to maintain the motor at a commanded RPM. During certainnavigations, such as during takeoff, landing, ascent, descent, orspecific maneuvers, increased torque may be desirable, even at theexpense of a maximum RPM and/or increased power consumption. However,during other aerial navigations, such as in-transit flight where the UAVis at a high altitude and navigating in a substantially horizontaldirection at a high speed, increased torque may not be as desirable as ahigher maximum RPM and decreased power savings. To increase the maximumRPM, the back-EMF may be reduced by, for example, increasing the gapdistance between the rotor magnets and the electromagnetic coils of thestator.

FIG. 5B depicts a top-down view of the motor stator 559 and rotor 554 ofFIG. 5A with the rotor in an expanded position, according to animplementation. As discussed below, the diameter of the rotor may bealtered from a contracted position, as illustrated in FIG. 5A, to anexpanded position, as illustrated in FIG. 5B, using a variety oftechniques. For example, the material used for form the rotor housingmay include a memory metal that transitions from the contracted positionto the expanded position in response to a force and/or energy, such asheat.

As illustrated in FIG. 5B, when the rotor 554 is in an expanded positionit has a second diameter (Diameter₂) that is larger than the firstdiameter (Diameter₁) illustrated in FIG. 5A. As a result, the gapdistance D_(G2) between the rotor magnets 562 and the electromagneticcoils 560 of the stator 559 increases. Likewise, the distance D_(R2)between the rotor magnets also increases. For example, the expandeddistance D_(R2) between a first rotor magnet 562-1 and a second rotormagnet 562-2 is larger than the contracted distance D_(R1) between thesame magnets when the rotor is in a contracted position, as illustratedin FIG. 5A.

By expanding the diameter of the rotor and thus increasing the gapdistance D_(G2) between the rotor magnets and the electromagnetic coils,the back-EMF decreases, which also results in the torque of the motordecreasing but the maximum RPM of the motor increasing. Likewise, thepower consumed by the motor to maintain a commanded RPM is less when therotor of the motor is in an expanded position. In some implementations,the difference in the gap distance between the rotor magnets and theelectromagnetic coils in a contracted position versus an expandedposition may only be a few millimeters. However, such a difference mayresult in a change in the back-EMF and/or other motor properties.

FIG. 6A depicts a motor in which the rotor 604 is in a contractedposition, according to an implementation. In this example, the exteriorof the rotor housing 605 includes a heating element 606. As a referencepoint when comparing FIG. 6A and FIG. 6B, when the rotor 604 is in thecontracted position (FIG. 6A), the rotor is approximately the same sizeas the base 608 of the motor. However, when the rotor 614 is in anexpanded position (FIG. 6B), the rotor 614 is larger than the base 618of the motor. In this example, the rotor is formed of a material thatexpands when heated. Accordingly, to cause the rotor to expand, theheating element 606/616 is energized, thereby transferring heat from theheating elements 606/616 into the material of the rotor housing, therebycausing the rotor to expand. When the rotor is to contract, the heatingelement is de-energized and the material of the rotor housing cools andcontracts.

A variety of techniques may be utilized to enable expansion andcontraction of the rotor. For example, the rotor housing 605 may beformed of one or more adaptable materials that may expand or contractunder certain conditions. For example, the adaptable material of therotor housing 605 may include one or more of a shape memory alloy, suchas, but not limited to, copper-aluminum-nickel alloy, nickel-titanium(NiTi), alloy, zinc alloy, copper alloy, gold alloy, iron alloy, etc.

In such examples, a force and/or energy may be applied to the adaptablematerial that causes the adaptable material, and thus the rotor, totransition from a contracted position to an expanded position, ortransition from an expanded position to a contracted position. Forexample, as discussed above with respect to FIGS. 6A and 6B, whenenergy, such as heat, is applied to the rotor housing, the adaptablematerial transitions from a contracted position (FIG. 6A) to an expandedposition (FIG. 6B). When the rotor housing transitions to the expandedposition, it causes the gap between the rotor magnets and theelectromagnetic coils of the stator to increase. When the adaptablematerial is not heated, the rotor returns to the contracted position(FIG. 6A) and the gap between the rotor magnets and the electromagneticcoil of the stator decrease.

The adaptable material may also be configured to respond to other formsof energy and either contract or expand in response to one or more otherforms of energy. Other forms of energy that may be utilized to cause anadaptable material of the rotor to transition from a contracted positionto an expanded position, and/or transition from the expanded position tothe contracted position include, but are not limited to, motion, sound,light, or heat.

In some implementations, other rotor housing configurations may beutilized to cause the rotor to expand and/or contract. For example, therotor housing may be configured to transition from a contracted positionto an expanded position in response to a force being applied to therotor housing. The force may include, but is not limited to, amechanical force, an electrical force, or a centrifugal force. Forexample, FIG. 6C depicts a rotor 624 with a rotor housing 625 in acontracted position, according to an implementation. In this example,the rotor housing 625 includes one or more channels 626 formed in thesurface of the rotor housing 625 and between the rotor magnets that aremounted to the interior of the rotor housing, as illustrated above. As areference point when comparing FIG. 6C and FIG. 6D, when the rotor 634is in the contracted position (FIG. 6C), the rotor is approximately thesame size as the base 628 of the motor. However, when the rotor 624 isin an expanded position (FIG. 6D), the rotor 634 is larger than the base638 of the motor. In this example, the channels 626/636 expand orcontract to cause the rotor housing 625/635 to expand or contract. Insome implementations, the transition of the rotor from a contractedposition (FIG. 6C) to an expanded position (FIG. 6D) may be done usingmechanical force and one or more actuators or motors that are includedin the rotor housing 625 that cause the channels to expanded and/orcontract.

In other implementations, the channels may be formed of one or morepre-tensioned springs that couple adjacent portions of the rotor housing205 together and allow conductivity between the rotor magnets. In such aconfiguration, as the RPM of the motor increases, the centrifugal forcecauses the springs to expand, thus expanding the channels 636 such thatthe rotor transitions from the contracted position to the expandedposition. Likewise, as the RPM of the motor decreases, the decrease incentrifugal force acting on the motor will allow the springs tocontract, transitioning the rotor from the expanded position to thecontracted positions. In other implementations, the channels may beformed of rubber, such as a conductive rubber, piezoelectric strips thatcan be actuated to expand or contract, springs, etc.

FIG. 7 is a flow diagram illustrating an example motor propertyadjustment process 700, according to an implementation. The exampleprocess 700 begins during operation of the aerial vehicle and determinesa desired motor property, as in 702. As discussed above, the describedimplementations allow alteration of motor properties during aerialnavigation and operation of the motor. For example, a first motorproperty may include a higher torque, a higher back-EMF, with a lowermaximum RPM, and potentially increased power consumption. A second motorproperty may include a lower torque, a lower back-EMF, with a highermaximum RPM, and potentially decreased power consumption.

A determination is then made as to whether the motor is to be adjustedfor higher speed, as in 704. If it is determined that the motor is to beadjusted for higher speed, the example process 700 causes the rotor totransition from a contracted position to an expanded position, as in706. As discussed above, causing the rotor to transition from acontracted position to an expanded position may be done by applying orremoving one or more forces (e.g., mechanical, electrical) or one ormore energies (e.g., heat, sound, light) to the rotor housing that causeor allow the rotor housing to expand.

If it is determined that the motor is not to be adjusted for higherspeed, a determination is made as to whether the motor is to be adjustedfor higher torque, as in 708. If it is determined that the motor is notto be adjusted for higher speed or higher torque, the example process700 returns to block 702 and continues. However, if it is determinedthat the motor is to be adjusted for higher torque, the example process700 causes the rotor to transition from the expanded position to thecontracted position, as in 710. As discussed above, causing the rotor totransition from an expanded position to a contracted position may bedone by applying or removing one or more forces (e.g., mechanical,electrical) or one or more energies (e.g., heat, sound, light) to therotor housing that cause or allow the rotor housing to contract.

The example process may be continually performed during operation of theaerial vehicle, performed periodically, performed at specific waypointsduring aerial navigation of the vehicle, and/or at any other time duringoperation. For example, each time a UAV transitions from vertical flightto horizontal flight, the example process 700 may be performed todetermine whether motor properties for motors of the aerial vehicle areto be altered. Such a determination may be based on, for example, thespeed of the aerial vehicle, the flight plan for the aerial vehicle, thealtitude of the aerial vehicle, other objects detected in the vicinityof the aerial vehicle, etc.

FIG. 8 is a block diagram illustrating an example UAV control system 830that may be used with a UAV, such as the UAV discussed above withrespect to FIG. 1. In various examples, the block diagram may beillustrative of one or more aspects of the UAV control system 830 thatmay be used to implement the various systems and methods discussedabove. In the illustrated implementation, the UAV control system 830includes one or more processors 802, coupled to a non-transitorycomputer readable storage medium 820 via an input/output (I/O) interface810. The UAV control system 830 may also include a motor controller 804,also known as an electronic speed control (ESC), power supply module806, and/or a navigation system 808. The UAV control system 830 may alsoinclude an engagement mechanism controller 812, a network interface 816,one or more input/output devices 818, and/or a motor property controller819.

In various implementations, the UAV control system 830 may be auniprocessor system including one processor 802, or a multiprocessorsystem including several processors 802 (e.g., two, four, eight, oranother suitable number). The processor(s) 802 may be any suitableprocessor capable of executing instructions. For example, in variousimplementations, the processor(s) 802 may be general-purpose or embeddedprocessors implementing any of a variety of instruction setarchitectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, orany other suitable ISA. In multiprocessor systems, each processor(s) 802may commonly, but not necessarily, implement the same ISA.

The non-transitory computer readable storage medium 820 may beconfigured to store executable instructions and/or data items accessibleby the processor(s) 802. In various implementations, the non-transitorycomputer readable storage medium 820 may be implemented using anysuitable memory technology, such as static random access memory (SRAM),synchronous dynamic RAM (SDRAM), nonvolatile/Flash-type memory, or anyother type of memory. In the illustrated implementation, programinstructions and data implementing desired functions, such as thosedescribed above, are shown stored within the non-transitory computerreadable storage medium 820 as program instructions 822, data storage824, and motor design and current patterns 826, respectively. In otherimplementations, program instructions, current patterns, flight plants,and/or other data may be received, sent, or stored upon different typesof computer-accessible media, such as non-transitory media, or onsimilar media separate from the non-transitory computer readable storagemedium 820 or the UAV control system 830. Generally speaking, anon-transitory, computer readable storage medium may include storagemedia or memory media such as magnetic or optical media, e.g., disk orCD/DVD-ROM, coupled to the UAV control system 830 via the I/O interface810. Program instructions and data stored via a non-transitory computerreadable medium may be transmitted by transmission media or signals suchas electrical, electromagnetic, or digital signals, which may beconveyed via a communication medium such as a network and/or a wirelesslink, such as may be implemented via the network interface 816.

In one implementation, the I/O interface 810 may be configured tocoordinate I/O traffic between the processor(s) 802, the non-transitorycomputer readable storage medium 820, and any peripheral devices, thenetwork interface or other peripheral interfaces, such as input/outputdevices 818. In some implementations, the I/O interface 810 may performany necessary protocol, timing or other data transformations to convertdata signals from one component (e.g., non-transitory computer readablestorage medium 820) into a format suitable for use by another component(e.g., processor(s) 802). In some implementations, the I/O interface 810may include support for devices attached through various types ofperipheral buses, such as a variant of the Peripheral ComponentInterconnect (PCI) bus standard or the Universal Serial Bus (USB)standard, for example. In some implementations, the function of the I/Ointerface 810 may be split into two or more separate components, such asa north bridge and a south bridge, for example. Also, in someimplementations, some or all of the functionality of the I/O interface810, such as an interface to the non-transitory computer readablestorage medium 820, may be incorporated directly into the processor(s)802.

The motor controller(s) 804 communicate with the navigation system 808and adjust the power of each motor to fly the UAV along a determinedflight path. As discussed above, in some implementations, the motorcontrollers send signals directly to the motor that cause differentcurrents to be applied to the motors. In other implementations, themotor controller(s) 804 send instructions to controllers integrated intothe motors that convert the information received from the motorcontroller(s) 804 to different current patterns for the motor based onthe motor design and/or position of the rotor of the motor. In stillother implementations, the motor controller(s) 804 may be omitted andthe motors of the UAV may communicate directly with the navigationsystem 808 and control operation of the respective motors.

The power supply module 806 may control the charging and any switchingfunctions associated with one or more power modules (e.g., batteries) ofthe UAV. The engagement mechanism controller 812 communicates withmechanisms (e.g., a servomotor) used to engage and/or disengage items tobe carried during flights. The network interface 816 may be configuredto allow data to be exchanged between the UAV control system 830 andother devices attached to a network, such as other computer systems. Invarious implementations, the network interface 816 may supportcommunication via wireless general data networks, such as a Wi-Finetwork. For example, the network interface 816 may supportcommunication via telecommunications networks such as cellularcommunication networks, satellite networks, and the like.

Input/output devices 818 may, in some implementations, include one ormore displays, image capture devices, thermal sensors, infrared sensors,time of flight sensors, accelerometers, pressure sensors, airflowsensors, speed sensors, vibration sensors, noise sensors, weightsensors, temperature sensors, etc. Multiple such input/output devices818 may be present and controlled by the UAV control system 830. Certainsensors may also be utilized to assist with navigation, landings,avoiding obstacles during flight, etc.

The motor property controller 819 communicates with and/or controlsmotor properties of the motors. As discussed above, motor properties ofone or more motors of the UAV may be altered by altering the diameter ofthe rotor of the motor. The motor property controller may communicationwith the navigation system 808 and determine desired motor properties.Based on the determined desired motor properties, the motor propertycontroller may send instructions to the motors and/or send energy (e.g.,heat) to the motors that cause the rotor of the motor to contract orexpand, thereby altering the motor property of the motor.

As shown in FIG. 8, the memory may include program instructions 822 thatmay be configured to implement the example processes and/orsub-processes described above. The data storage 824 may include variousdata stores for maintaining data items that may be provided fordetermining flight plans, landing, etc. In some implementations, datastores may be physically located in one memory or may be distributedamong two or more memories.

Those skilled in the art will appreciate that the UAV control system 830is merely illustrative and is not intended to limit the scope of thepresent disclosure. In particular, the computing system and devices mayinclude any combination of hardware or software that can perform theindicated functions. The UAV control system 830 may also be connected toother devices that are not illustrated, or instead may operate as astand-alone system. In addition, the functionality provided by theillustrated components may in some implementations be combined in fewercomponents or distributed in additional components. Similarly, in someimplementations, the functionality of some of the illustrated componentsmay not be provided and/or other additional functionality may beavailable.

Those skilled in the art will also appreciate that, while various itemsare illustrated as being stored in memory or storage while being used,these items or portions of them may be transferred between memory andother storage devices for purposes of memory management and dataintegrity. Alternatively, in other implementations, some or all of thesoftware components may execute in memory on another device andcommunicate with the illustrated UAV control system 830. Some or all ofthe system components or data structures may also be stored (e.g., asinstructions or structured data) on a non-transitory,computer-accessible medium or a portable article to be read by anappropriate drive, various examples of which are described above. Insome implementations, instructions stored on a computer-accessiblemedium separate from the UAV control system 830 may be transmitted tothe UAV control system 830 via transmission media or signals such aselectrical, electromagnetic, or digital signals, conveyed via acommunication medium such as a wireless link. Various implementationsmay further include receiving, sending, or storing instructions and/ordata implemented in accordance with the foregoing description upon acomputer-accessible medium. Accordingly, the techniques described hereinmay be practiced with other UAV control system configurations.

Those skilled in the art will appreciate that, in some implementations,the functionality provided by the processes and systems discussed abovemay be provided in alternative ways, such as being split among moresoftware modules or routines or consolidated into fewer modules orroutines. Similarly, in some implementations, illustrated processes andsystems may provide more or less functionality than is described, suchas when other illustrated processes instead lack or include suchfunctionality respectively, or when the amount of functionality that isprovided is altered. In addition, while various operations may beillustrated as being performed in a particular manner (e.g., in serialor in parallel) and/or in a particular order, those skilled in the artwill appreciate that, in other implementations, the operations may beperformed in other orders and in other manners. Those skilled in the artwill also appreciate that the data structures discussed above may bestructured in different manners, such as by having a single datastructure split into multiple data structures or by having multiple datastructures consolidated into a single data structure. Similarly, in someimplementations, illustrated data structures may store more or lessinformation than is described, such as when other illustrated datastructures instead lack or include such information respectively, orwhen the amount or types of information that is stored is altered. Thevarious methods and systems as illustrated in the figures and describedherein represent example implementations. The methods and systems may beimplemented in software, hardware, or a combination thereof in otherimplementations. Similarly, the order of any method may be changed andvarious elements may be added, reordered, combined, omitted, modified,etc., in other implementations.

From the foregoing, it will be appreciated that, although specificimplementations have been described herein for purposes of illustration,various modifications may be made without deviating from the spirit andscope of the appended claims and the elements recited therein. Inaddition, while certain aspects are presented below in certain claimforms, the inventors contemplate the various aspects in any availableclaim form. For example, while only some aspects may currently berecited as being embodied in a particular configuration, other aspectsmay likewise be so embodied. Various modifications and changes may bemade as would be obvious to a person skilled in the art having thebenefit of this disclosure. It is intended to embrace all suchmodifications and changes and, accordingly, the above description is tobe regarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. An outrunner brushless direct current (DC) motor,comprising: a stator including a plurality of electromagnetic coils; arotor including a rotor housing that substantially encompasses thestator, the rotor housing including an adjustable portion that causesthe rotor to have a first diameter at a first temperature and furthercauses the rotor to have a second diameter at a second temperature, thesecond diameter being different than the first diameter; and a heatingelement configured to alter a temperature of the adjustable portion totransition between the first temperature and the second temperature suchthat the rotor transitions between the first diameter and the seconddiameter.
 2. The outrunner brushless DC motor of claim 1, furthercomprising: a plurality of magnets coupled to an interior surface of therotor and spaced a first distance from the plurality of electromagneticcoils when the rotor has the first diameter and spaced a second distancefrom the plurality of electromagnetic coils when the rotor has thesecond diameter.
 3. The outrunner brushless DC motor of claim 2,wherein: the second diameter is larger than the first diameter; and thesecond distance is larger than the first distance, such that aback-electromotor force (back-EMF) of the motor is decreased when therotor has the second diameter.
 4. The outrunner brushless DC motor ofclaim 1, wherein: the adjustable portion includes a shape memory alloy.5. The outrunner brushless DC motor claim 4, wherein the shape memoryalloy includes at least one of a copper-aluminum-nickel alloy, anickel-titanium (NiTi) alloy, a zinc alloy, a copper alloy, a goldalloy, or an iron alloy.
 6. A motor, comprising: a stator including aplurality of electromagnetic coils; and a rotor that substantiallyencompasses the plurality of electromagnetic coils, the rotor includinga portion that is adjustable during operation of the motor between: acontracted position in which the rotor has a first diameter, and anexpanded position in which the rotor has a second diameter that islarger than the first diameter; and an element associated with theportion of the rotor and configured to apply an energy to the portion ofthe rotor; and wherein the rotor transitions between the first diameterand the second diameter in response to the energy.
 7. The motor of claim6, wherein: the motor has a first torque, a first back-electromotorforce (back-EMF), and a first maximum revolutions per minute (RPM) inthe contracted position; the motor has a second torque, a secondback-EMF, and a second maximum RPM in the expanded position; the firsttorque is higher than the second torque; the first back-EMF is higherthan the second back-EMF; and the first maximum RPM is lower than thesecond maximum RPM.
 8. The motor of claim 6, further comprising: aplurality of magnets coupled to an interior of the rotor, the pluralityof magnets separated from the plurality of electromagnetic coils by afirst distance when the rotor is in the contracted position andseparated from the plurality of electromagnetic coils by a seconddistance when the rotor is in the expanded position.
 9. The motor ofclaim 6, wherein the portion of the rotor expands when a force isapplied to the portion such that the rotor also transitions from thefirst diameter to the second diameter when the force is applied.
 10. Themotor of claim 9, wherein the force is at least one of a mechanicalforce, an electrical force, or a centrifugal force.
 11. The motor ofclaim 6, wherein the element comprises at least one of a heatingelement, a shape memory alloy, a piezoelectric strip, or an actuator.12. The motor of claim 6, wherein the energy is at least one of amotion, a sound, a light, or a heat.
 13. The motor of claim 6, wherein:the portion of the rotor includes a plurality of channels formed in asurface of the rotor; and the plurality of channels expand in responseto the energy, thereby causing the rotor to transition from the firstdiameter to the second diameter.
 14. The motor of claim 6, wherein: theelement is a heating element and the energy is heat; and the rotortransitions from the first diameter to the second diameter in responseto the heat.
 15. A computer-implemented method, comprising: undercontrol of one or more computing systems configured with executableinstructions, causing, during operation of a motor, a rotor of the motorto have a first diameter such that the motor has a first motor property;determining, during operation of the motor, that the motor is to have asecond diameter such that the motor has a second motor property; and inresponse to determining that the motor is to have the second motorproperty, causing, during operation of the motor, an element to apply orremove an energy with respect to the rotor such that the rotor of themotor adjusts from the first diameter to the second diameter.
 16. Thecomputer-implemented method of claim 15, wherein: the first motorproperty includes a first torque and a first maximum revolutions perminute (RPM); the second motor property includes a second torque and asecond maximum RPM; the second torque is less than the first torque; andthe second maximum RPM is higher than the first maximum RPM.
 17. Thecomputer-implemented method of claim 15, further comprising:determining, during operation of the motor, that the motor is to havethe first diameter such that the motor has the first motor property; andcausing, during operation of the motor, the element to apply or removethe energy with respect to the rotor such that the rotor of the motoradjusts from the second diameter to the first diameter.
 18. Thecomputer-implemented method of claim 15, wherein a back-electromotorforce (back-EMF) decreases when the rotor of the motor expandsresponsive to application or removal of the energy with respect to therotor.
 19. The computer-implemented method of claim 15, wherein adistance between a plurality of electromagnetic coils of a stator of themotor and a plurality of magnets of the rotor of the motor is adjustedresponsive to adjustment of the rotor of the motor between the firstdiameter and the second diameter.
 20. The computer-implemented method ofclaim 15, wherein a back-electromotor force (back-EMF) of the motorchanges in response to a change in a diameter of the rotor of the motor.